1. Field of the Invention
This invention relates, in general, to the field of semiconductor manufacture. In particular, it relates to monitoring the conditions of post chemical mechanical polishing processing.
2. Description of Related Art
Fabrication of semiconductor integrated circuits (IC) is a complicated multi-step process for creating microscopic structures with various electrical properties to form a connected set of devices. As the level of integration of ICs increases, the devices become smaller and more densely packed, requiring more levels of photolithography and more processing steps. As more layers are built up on the silicon wafer, problems caused by surface non-planarity become increasingly severe and can impact yield and chip performance. During the fabrication process, it may become necessary to remove excess material in a process referred to as planarization.
A common technique used to planarize the surface of a silicon wafer is chemical mechanical polishing (CMP). CMP involves the use of a polishing pad affixed to a circular polishing table and a holder to hold the wafer face down against the rotating pad. A slurry containing abrasive and chemical additives are dispensed onto the polishing pad.
The wafer and polishing pad rotate relative to each other. The rotating action along with the abrasive and chemical additives of the slurry results in a polishing action that removes material from the surface of the wafer. Protrusions on the surface erode more efficiently than recessed areas leading to a flattening or planarization of the wafer surface. Following CMP, the wafer must be cleaned of any CMP and slurry residue. Any residue remaining on the wafer can cause shorts to the IC devices.
A known method of post-CMP processing in the prior art utilizes brush cleaning technology such as the Dai Nippon Screen Model No. SP-W813-AS (DNS brush cleaner). The DNS brush cleaner cleans the wafer using a combination of rinsing, megasonic rinsing, and brush cleaning.
The wafers are loaded into a wet environment, usually water, then transported through a series of cleaning chambers for the brush cleaning cycle. The brush cleaning cycle involves rotating the wafer at high speed, about 1500 rpm, while a jet of deionized water is sprayed on the wafer to dislodge any loose debris from CMP, and the wafer is brushed with a foam brush.
During the brush cleaning cycle, the brush is first placed over the center of the wafer. The brush contacts the backside of the wafer, presses down on the wafer, and moves at a constant height and pressure to the periphery of the wafer in one stroke. The brush then retracts from the wafer and the whole cycle is repeated. The following chamber then brushes the top side of the wafer. After the brushing cycles, the wafer is deposited in the spin/rinse/dry chamber and unloaded dry.
The problem associated with the DNS brush cleaner is the inability of the machine to recognize whether the brush has fallen off and is not making contact with the wafer. A tool user cannot actually see that the brush has fallen off the brush holder since the cleaning process is within an enclosed chamber. Also problems arise when the brush""s contact pressure is improperly set during maintenance set up or during the brush cleaning cycle. The silent failure of the brush to properly clean the wafer is difficult to detect until the next inspection point. At that time, several wafer lots have gone through the brush cleaner and would require reworking the lots, a costly solution.
U.S. Pat. No. 4,561,214 issued to Inoue on Dec. 31, 1985, discloses a motor driven abrading tool having a sensor to monitor electrical resistance between the contact surfaces of the tool and the object being abraded. As the contact resistance increases with the decrease of the urging pressure of the tool, the current from the electrical source is reduced and the voltage drops. A control circuit responds to the drop in voltage to control the rpm of the abrading tool motor or to control the torque on the tool to change the contact pressure of the tool on the object being abraded. This patent does not disclose or suggest monitoring the torque as a means to detect the presence of the abrading tool but rather as a means to adjust the contact pressure of the tool.
U.S. Pat. No. 4,757,566 issued to Field et al. on Jul. 19, 1988, discloses an automatic tool torque compensator for a surface maintenance machine. Once an operator of the machine determines what level of tool torque is desired, the tool torque compensator automatically maintains the tool torque at the desired setting although the surface maintained may vary in elevation or texture. The tool torque compensator achieves the torque compensation by using an electrical signal representative of current load for the tool motor and a signal representative of the desired level of tool torque and integrates the two signals to raise or lower the surface maintenance tools which change their pressure against the surface being maintained. However, this patent discloses adjusting the contact pressure of the tool on the surface being maintained and does not disclose or suggest using the torque value as an input to detecting the presence of the tool itself.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a post-CMP brush cleaner for semiconductor wafers having a means for ensuring proper wafer cleaning.
It is another object of the present invention to provide a method of ensuring proper wafer cleaning during post-CMP processing.
A further object of the invention is to provide a method of ensuring proper wafer cleaning during post-CMP processing utilizing a DNS brush cleaner.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, a semiconductor wafer cleaning tool comprising a cleaning chamber; a rotatable fixture within the cleaning chamber for securing and rotating a semiconductor wafer during cleaning; a brush cleaner within the cleaning chamber for cleaning a surface of the wafer; and a means for detecting the presence of the brush cleaner during wafer cleaning.
The brush cleaner comprises a rotating brush and a brush arm adapted to move the rotating brush across the surface of the rotating wafer.
The semiconductor wafer cleaning tool further includes a servo motor for rotating the rotatable fixture. The servo motor further includes a servo controller connected to the servo motor. The servo controller interfaces with a processor such that the processor is adapted to detect the absence of the brush cleaner to a tool user based on an output of the servo controller.
The detecting means of the cleaning tool comprises a torque monitor linked to the servo controller to calculate a torque on the rotating wafer. The torque monitor is adapted to determine the presence of the brush cleaner on the rotating wafer during cleaning of the wafer. The presence of the brush cleaner on the rotating wafer during cleaning of the wafer is detemined by calculating a troque on the rotating wafer when contacted with the brush cleaner, the torque being greater than a torque on the rotating wafer alone.
The torque monitor sends a signal to the servo controller when the torque on the rotating wafer when contacted with the brush cleaner is substantially equal to the torque on the rotating wafer alone, and the servo controller sends a signal to the servo motor to cease rotating the rotatable fixture. The servo controller sends an analog signal to the torque monitor, the analog signal is converted to a digital signal by the torque monitor.
Alternatively, the detecting means comprises a voltage sensor linked to the servo motor controller to display the voltage of the servo motor. The voltage sensor is adapted to determine the presence of the brush cleaner on the rotating wafer during cleaning of the wafer.
In another aspect, the present invention relates to a post-CMP cleaning tool comprising a cleaning chamber; a rotatable semiconductor wafer fixture within the cleaning chamber for securing and rotating a wafer during cleaning; a motor linked to the rotatable fixture for rotating the fixture; a motor controller linked to the motor; a brush cleaner within the chamber having a rotating brush, and a brush arm adapted to move the rotating brush across a surface of a rotating wafer; a torque monitor linked to the motor controller for detecting the presence of the brush cleaner during wafer cleaning.
The torque monitor of the post-CMP cleaning tool calculates a torque on the rotating wafer. The torque monitor is adapted to determine the absence of the brush cleaner on the rotating wafer during cleaning of the wafer. The torque monitor determines the absence of the brush cleaner on the rotating wafer during cleaning by calculating a torque on the rotating wafer when contacted with the brush cleaner, the torque being greater than a torque on the rotating wafer alone. The torque monitor signals the motor controller when the absence of the brush cleaner is detected, the motor controller sending a signal to the motor to cease rotating the fixture.
In still another aspect, the present invention relates to a method of cleaning a semiconductor wafer following CMP comprising the steps of: (a) providing a semiconductor wafer in need of removing CMP residue; (b) providing a wafer cleaning apparatus having at least one cleaning chamber; a brush cleaner; a wafer rotating device having a motor to rotate the wafer during cleaning; and means for monitoring the presence of the brush cleaner; (c) loading the wafer into the at least one cleaning chamber; (d) rotating the wafer; (e) brushing the rotating wafer with the brush cleaner; and (f) monitoring the presence of the brush cleaner to ensure cleaning of the wafer.
The step of monitoring the presence of the brush cleaner is adapted to measuring the torque on the rotating wafer as the brush cleaner brushes the rotating wafer. Alternatively, the step of monitoring the presence of the brush cleaner is adapted to measuring the voltage on the motor of the wafer rotating device.
In yet another aspect, the present invention relates to the method of monitoring the status of the cleaning process in a semiconductor wafer brush cleaning tool, comprising the steps of: (a) providing a semiconductor wafer in need of cleaning; (b) providing a brush cleaner; (c) rotating the semiconductor wafer; (d) obtaining a first torque on the rotating wafer; (e) contacting the brush cleaner to the rotating wafer; (f) cleaning the rotating semiconductor wafer by moving the brush cleaner over a surface of the wafer; (g) obtaining a second torque on the rotating wafer as the brush cleaner moves over the surface of the rotating wafer; and (h) comparing the first torque and the second torque to determine the presence of the brush cleaner.
The method may further include the step of signifying when the second torque is substantially equal to the first torque. The second torque on the rotating wafer is greater than the first torque when the brush cleaner is in contact with the semiconductor wafer. During the step of obtaining a second torque on the rotating wafer, the second torque is substantially equal to the first torque signifies that the brush cleaner is absent from the surface of the wafer.
The method may further include the step of discontinuing cleaning of the wafer when the brush cleaner is absent from the surface of the wafer.
The step of rotating the wafer is performed by a servo motor. The servo motor is controlled by a servo controller such that the controller regulates rotating of the wafer. During the step of obtaining a second torque on the rotating wafer, wherein the second torque is substantially equal to the first torque, the servo controller sends a signal to the servo motor to cease rotating the wafer.
The servo controller interfaces with a torque monitor adapted to electrically receive analog signals from the servo controller and convert the analog signals to digital signals for further digital signal processing. The digital signal processing includes calculating and comparing torques.